Microbial succession of Debaryomyces hansenii strains during the production of Danish surfaced-ripened cheeses.

Surface-ripened cheeses of the Danbo type were analyzed for the presence of yeasts with special emphasis on Debaryomyces hansenii. Samples were taken from pasteurized milk, brine, and inoculation slurries and from cheese surfaces during ripening at a Danish dairy. D. hansenii was found to be the dominant yeast species throughout the ripening period, whereas other yeast species such as Trichosporon spp., Rhodotorula spp., and Candida spp. were found in minor concentrations during early stages of cheese ripening. Mitochondrial DNA RFLP was used to show that several strains of D. hansenii were present from the onset of ripening. Thereafter, a microbial succession among the strains took place during the ripening. After 3 d of ripening, only one strain was found. This particular strain was found to be dominant in 16 additional batches of surface-ripened cheeses. We investigated the cause of the observed microbial succession by determining the variation in strains with regard to their ability to grow on lactate and at different pH and NaCl concentrations. The strains were shown to vary in their ability to grow on lactate. In a full factorial design at three levels with factor levels close to the actual levels on the cheese surface, differences in pH and NaCl tolerances were observed. The dominant strain was found to be better adapted than other strains to the environmental conditions existing in surface-ripened cheeses during production [e.g., lactate as the main carbon source, pH 5.5 to 6.0 and NaCl concentrations of 7 to 10% (wt/vol)].

Associated mycoflora of rye bread.

Penicillium roqueforti (27%), P. corylophilum (20%) and Eurotium species (15%) made up the important mycoflora associated with rye bread on 3425 identified fungi isolates. These fungi were dominant as spoilers of packaged rye bread in almost every month of a 4 year investigation. Penicillium decumbens (3%), Paecilomyces variotti (8%) and Aspergillus flavus (5%) were found more rarely, but were the major species found over periods of a few months. Penicillium commune (5%), P. solitum (4%), A. niger (4%) and Mucor species (2%) were a constant, but small, part of the total mycoflora of rye bread. Identification of the fungi in the production environment in a rye bread factory showed the locality of potential contamination sources. Elimination of the contamination sources by improved cleaning and disinfection procedures quickly resulted in a significant reduction in the frequency of mould growth in the packaged rye bread.